The present invention relates to a security device (and method for its production) for use for example on security documents and documents of value such as banknotes, cheques, bonds, certificates, fiscal stamps, tax stamps, vouchers, and brand protection.
It is well known within security printing to use luminescent materials to produce security features. Luminescent materials are known to those skilled in the art to include materials having fluorescent or phosphorescent properties. It is also well known to use other materials that respond visibly to invisible radiation such as photochromic materials and thermochromic materials.
An example of a luminescent feature utilised within security printing can be found in EP-A-253543. This case describes a lustrous metallic ink having differing appearances in visible and UV light. Such metallic fluorescent inks have proved very successful and are widely used on security documents. They provide a metallic ink clearly visible to the public with the additional security that fluorescence provides. The ink is typically printed in a discreet area and has a single colour under UV illumination.
A different type of feature is described in GB-A-1407065, which makes use of metamerism. The case describes the use of metameric pairs of inks appearing essentially the same under a first illuminant, such as natural sunlight, but different under a second illuminant having a different spectral energy distribution, for example produced by a tungsten filament lamp. The embodiments described within the patent are all designed to display metameric properties under differing visible light conditions.
WO-A-9840223 describes a method of printing an image that is invisible under normal lighting conditions but visible under UV illumination. The image visible under UV illumination comprises at least two different colours. The image visible under UV illumination may be the same as another image visible elsewhere on the document under normal lighting condition e.g. a portrait or photograph. It is a requirement of this case that the image viewable under UV illumination is not visible under normal lighting conditions, indeed the inks used are said to be invisible.
WO-A-0078556 describes a security document having both visible and invisible information characterised in that the invisible information is personalised. Particular examples are cited as printing invisible bar codes onto driving licences, passports and other documents intended to confirm a persons identity.
EP-A-1179807 describes an anti-fraud device for documents consisting of a support and at least two printed motifs affixed to the said support, distinguished in that one of the motifs contains an ink that responds to a given wavelength by emitting a specific colour and one other motif contains an ink that reacts to the same wavelength by emitting the same colour but also reacts to a second wavelength by emitting another colour.
EP-A-1179808 describes an anti-fraud device for documents consisting of a support and at least two printed motifs affixed to the said support, distinguished in that one of the motifs contains a first ink that responds to ultraviolet radiation of a given wavelength by emitting a specific colour and one other motif contains a second ink that responds to ultraviolet radiation of the same wavelength by emitting the same colour as the first ink, and the two inks, when subjected to ultraviolet radiation of a second wavelength, emit different colours from each other.